1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet having superior magnetic and surface film characteristics.
2. Description of the Related art
Grain-oriented electrical steel sheets are mainly used as an iron core for transformers, generators and other electrical equipment, and must have a good surface film as well as good magnetic characteristics including magnetic exciting and watt-loss characteristics.
The magnetic characteristics of a grain-oriented electrical steel sheet are obtained through a Goss-orientation having a {110} plane parallel to the sheet surface and a &lt;001&gt; axis in the rolling direction, which is established by utilizing a secondary recrystallization occurring during a final annealing step.
To induce a secondary recrystallization to a substantially effective extent, fine precipitates of AlN, MnS, MnSe or the like, which act as an inhibitor for suppressing the growth of primary-recrystallized grains, must exist up to a temperature range in which a secondary recrystallization is effected during a final annealing. To this end, an electrical steel slab is heated to a high temperature of 1350.degree. to 1400.degree. C., to ensure a complete dissolution of inhibitor-forming elements such as Al, Mn, S, Se, and N. The inhibitor-forming elements completely dissolved in a steel slab are precipitated as fine precipitates such as AlN, MnS, and MnSe during the annealing of a hot-rolled sheet, or during an intermediate annealing carried out between cold rolling steps before a final cold rolling.
This process also has a problem in that a large amount of molten scale is formed during the heating of a slab at such a high temperature, and this makes frequent repairs to the heating furnace necessary, raises maintenance costs, causes a lowering of the facility operating rate, and leads to a higher consumption of energy.
To solve the above problem, research has been carried out into the development of a process for producing a grain-oriented steel sheet in which a lower slab heating temperature can be used.
For example, Japanese Unexamined Pat. Publication (Kokai) No. 52-24116 proposed a process in which a lower slab heating temperature of from 1100.degree. to 1260.degree. C. can be utilized by using an electrical steel slab containing Al and other nitride forming elements such as Zr, Ti, B, Nb, Ta, V, Cr, and Mo.
Japanese Unexamined Pat. Publication (Kokai) No. 59-190324 also proposed a process in which a slab heating temperature not exceeding 1300.degree. C. can be utilized by using an electrical steel slab having a reduced carbon content of 0.01% or less and selectivity containing S, Se, Al, and B, and by a pulse annealing in which, during the primary recrystallization annealing after cold rolling, the steel sheet surface is repeatedly heated to a high temperature at short intervals.
Japanese Examined Pat. Publication (Kokoku) No. 61-60896 proposed another process in which a slab heating temperature lower than 1280.degree. C. can be utilized by using an electrical steel slab having a Mn content of from 0.08 to 0.45% and a S content of 0.007% or less, to produce a reduced value of the [Mn] [S] product, and containing Al, P, and N.
Nevertheless, in these conventional processes, when used for producing a grain-oriented electrical steel sheet, a problem arises in that the surface glass film of a final product sheet occasionally is marred by a defect known as "frost-spotted pattern or "bare spots".